The present invention relates to an optical recording medium such as an optical disc on which high density information has been recorded by means of a laser beam. The present invention also relates to a method for reproducing information recorded on such optical recording medium.
In recent years, proposals surfaced for a Super Image Dissection Reproducing Technique where high density information may be reproduced with the use of pits smaller than a laser beam spot For instance, there has been a suggestion that a chalcogen amorphous material such as Sb.sub.2 Se.sub.3 be employed, so that high density information may be reproduced by making use of a phenomenon called Reflectance Change which is caused by a reversible phase change between an amorphous state and a crystallized state (Japanese Patent Application Laid-open 3-292632).
According to this prior art, a film of a phase change material is formed on a transparent substrate in which signal pits have already been formed in accordance with information signals. When the laser beam is applied to the phase change material, the rising temperature of the phase change material within the applied laser beam spot causes the reflectance to rise so as to form a high reflectance area (amorphous state). In reproduction, only pits within the high reflectance area are detected, and high density reproduction may be effected by making use of Reflectance Change, i.e., a fact that an information reproducing area is a high reflectance area and other areas are low reflectance areas.
Namely, the technique disclosed in the above prior art is useful for an optical disc of the ROM (Read Only Memory) type which includes a transparent substrate having signal pits formed in advance, and a single layer of phase change material formed on the transparent substrate.
However, there has not been suggested a technique useful for other types of optical recording medium such as the Write Once, the Read Many type, or the Rewritable type: with which can record information and reproduce the information at any desired time.
Moreover, even with the technique being useful only for the ROM type optical disc, the reflectance of areas (masked areas) other than the information reproducing area are required to be sufficiently lower than that of the information reproducing area, but with respect to the focus servo, it is required that the masked areas should have a high reflectance.